The DoD has been deploying Global Positioning System (GPS) satellites for military and civil use beginning in 1978. Today, there are 25 GPS satellites approximately uniformly spaced around the earth. From any earth location one expects to be able to track typically five to seven GPS satellites at any given time. In fact there are times when 10 or more satellites can be tracked simultaneously. These satellites broadcast signals on two frequencies L1 and L2. L1 and L2 are, respectively 154 and 120 times the basic clock frequency of 10.23 MHz. The L1 and L2 carrier signals can be tracked by a GPS receiver providing few-millimeter-precision "carrier" ranges between the receiver and satellite. By multiplying the carrier by +1 or -1 at precise points in the signal stream, the GPS satellite produces a coded signal which a GPS receiver expects. The code ranges have precision in the range of 1 decimeter to a few meters (depending on receiver hardware) but are effectively unambiguous; the carrier ranges have few-millimeter precision but are ambiguous. The strengths of code and carrier ranges need to be combined for this apparatus to achieve both accuracy and robustness.
When a user deploys two GPS receivers, it is possible to achieve centimeter accuracy in a relative sense between the two receivers. Typically one receiver is a fixed reference receiver and the second is at a fixed unknown site or is in motion and its instantaneous position (i.e., unknown) is desired with centimeter accuracy in three dimensions (e.g., North, East, Up). Typical hardware for the implementation of the instant invention's method is shown and described in prior U.S. Pat. No. 5,177,489 to Hatch entitled "Pseudolite-Aided Method for Precision Kinematic Positioning" and U.S. Pat. No. 5,148,179 to Allison entitled "Differential Position Determination Using Satellites" which are hereby incorporated by reference.
Prior references that deal with precise (i.e., centimeter-level) positions that can be sequentially determined based on carrier phase kinematic positioning methods is taught in U.S. Dept. of Commerce NOAA Technical Memorandum NOS NGS-55, May 1991 by Benjamin W. Remondi entitled "Kinematic GPS Results Without Static Initialization." This reference teaches of an approximate initial positioning by meter-level differential GPS solutions that is used in the instant invention, but does not sufficiently teach or suggest the method and apparatus herein.
U.S. Pat. No. 5,177,489 to Hatch discloses a method for precision kinematic positioning of a remote receiver using a ground based psuedolite in addition to orbital satellites wherein measurements from a minimum of satellites are used to determine an initial set of potential solutions to the portion of the remote receivers antenna. Redundant measurements from additional satellites are then used to progressively reduce the number of potential solutions to close to one. Limitations of this patent include: (1) the need for a psuedolite as part of the system hardware which isn't required by the instant invention where a minimum of four satellites is all that is required to make fast and precise measurements; (2) the technique used in this patent cannot deal with many high speed real-time applications, whereas the instant invention's technique is more robust in eliminating false potential locations in a grid due to the fact that the required satellite carrier signal data input to the remote roving and reference signal receivers in any required epoch is shorter due to the fewer number of satellites required and no redundant satellites are required for precise location measurements; (3) the technique used in this patent cannot resolve carrier ambiguities solely with four GPS satellites or with a combination of GPS satellites and pseudolites which add to four whereas the instant invention can; (4) the technique used in this patent depends upon a wide lane which is defined as the difference in the L1 and L2 carrier measurements forming L1-L2 whereas the instant invention does not require this feature and creates a greater signal utilization by forming independent L1 and L2 grids for potential solutions of the location; (5) the method of forming four parallel planes to create the set of grid point candidates can be a limiting factor when any configuration of four satellites has high positional dilution of precision (PDOP) whereas the instant invention herein uses a method called Ambiguity Resolution Using Trial Position (ARUTP) which makes for a more robust method; (6) the patent has limited range as to distance between the remote roving and reference receivers due to the fact of using psuedolite(s) which are located on the earth whereas the instant invention uses at least four satellites that are above the earth at any time and (7) the patent requires continuous L2 carrier phase data for a dual frequency method whereas the instant invention requires only fractional phase for L2.
U.S. Pat. No. 5,148,179 to Allison discloses another method for precision kinematic positioning of a remote receiver using only orbital satellites which are at least four in number wherein psuedorange double differences are formed, between each of the two receivers and each of a first satellite and the other three satellites derived from the either of the two signal frequencies. Limitations of this patent include: (1) the technique used in this patent depends upon a wide lane formed by the difference in L1 and L2 carrier measurements forming L1-L2 whereas the instant invention does not require this feature and creates a greater signal utilization by forming an L1 and L2 grid; and (2) the technique used in this patent can initialize carrier integer ambiguities based on only four satellites if the initial differential GPS determination achieves better than half a wide lane whereas the instant invention does not require this limitation in a situation where only four satellites are present.
U.S. Pat. No. 5,296,861 to Knight entitled "Method and Apparatus For Maximum Likelihood Estimation Direct Integer Search In Differential Carrier Phase Attitude Determination Systems" discloses a statistical method for rapid resolution of integer ambiguities in measured GPS carrier phase data. Limitations of this patent include it being an attitude system requiring a priori knowledge of the distance from the reference receiver to the remote receiver which requires that the distance between reference and remote receiving antennas be extremely small whereas the instant invention allows for much greater distances.
Accordingly, there is still need for improvement in this field to provide a method and apparatus that i) reduces the time needed to eliminate false position solutions in the carrier phase ambiguity resolution of GPS signals, ii) increases the operating range between the remote and reference receiving antenna, iii) reduces the number of false initializations, and iv) minimizes the number of satellites required in the initialization phase while still allowing for initialization of a moving remote receiver.
OBJECTS AND SUMMARY OF THE INVENTION
The invention provides a method for use in a global positioning system comprising hardware and software that converts measurements taken from two GPS receivers into centimeter-level-accuracy positions in either real-time or post-processed mode. A very important aspect of the invention is its' real time capabilities where carrier integer ambiguities can be initialized for a moving OTF platform applications, i.e. a remote roving receiver, without ever having to stop. For static applications, the method can be used identically. The invention requires, as a minimum, measurements of the code and carrier on at least the L1 GPS signal (i.e., both code and carrier ranges). The invention performs significantly better if the L1 code and carrier observations are accompanied by codeless L2-squared carrier ranges.
The invention performs substantially superior to L2-squared when the L1 code and carrier are augmented with either codeless or P code full-wavelength L2 carrier ranges. The invention performs at its best when the full suite of independent GPS observable are available (i.e., full-wavelength L1 and L2 carrier ranges plus independent L1 and L2 code ranges, whether P code or not). The carrier measurements can be accomplished with only four satellites to make measurements from.
Within the method, both the reference receiver and the remote receiver can be moving or fixed, in any combination with respect to the earth's surface. For improved performance, the invention permits the imposition of a priori constraints such that fewer satellites are required for the initialization period.